BACKGROUND
[0001] This disclosure generally relates to materials and articles for service in high-temperature
applications such as, for example, turbomachinery. More specifically, this disclosure
relates to methods for removing coatings from ceramic-matrix composite substrates.
[0002] Ceramic matrix composite (CMC) materials offer the potential for higher operating
temperatures than do metal alloy materials due to the inherent high-temperature material
properties of ceramic materials. In applications such as gas turbine assemblies, this
capability may be translated into a reduced cooling requirement which, in turn, may
result in higher power, greater efficiency, and/or reduced emissions from the machine.
However, CMC materials that include significant amounts of silicon-bearing materials,
such as silicon carbide or silicon nitride, are susceptible to attack and rapid recession
by water vapor at elevated service temperatures. Environmental barrier coatings (EBC)
have been developed to inhibit this degradation mechanism.
[0003] During service, one or more portions of the EBC may become damaged, but because CMC
components typically are expensive, removing the damaged EBC and re-coating the used
CMC component is economically advantageous over replacing the entire component. EBC's
can be removed by mechanical processes such as grit blasting, but such operations
may lead to damage of the CMC substrate due to the desirably strong bonding between
CMC and EBC.
[0004] There is thus a need in the industry for methods for removing coatings such as EBC
from CMC substrates without unduly damaging the CMC material.
BRIEF DESCRIPTION
[0005] Embodiments of the present invention are provided to meet this and other needs. One
embodiment is a method for removing a coating from an article. The method includes
heating an article to a processing temperature. The article includes a first material
in contact with a second material, the first material comprising silicon, and the
second material comprising an oxide comprising silicon. The heating is performed in
an environment having a partial pressure of oxygen that is less than an equilibrium
partial pressure of oxygen for chemical equilibrium between the first material and
the second material at the processing temperature.
[0006] Another embodiment is a method for removing a coating from an article. The method
includes heating the article to a processing temperature at least about 1200 degrees
Celsius in a vacuum having a total pressure less than about 10
-2 torr (1.3 Pa). The article includes a substrate comprising a ceramic matrix composite,
the composite comprising silicon carbide, silicon nitride, or a combination comprising
one or both of the aforementioned; a first material disposed over the substrate and
including elemental silicon, an alloy comprising elemental silicon, a silicide, or
a combination comprising one or more of the aforementioned; a second material in contact
with the first material, the second material comprising silica, a silicate, or a combination
comprising one or both of the aforementioned, and a third material disposed over the
second material, the third material comprising a rare earth silicate, an aluminosilicate,
zirconia, or a combination comprising one or more of the aforementioned. The article
is heated at the processing temperature in the described environment until a desired
degree of reaction between first material and second material has occurred, and then
the third material is removed from substrate.
DRAWINGS
[0007] These and other features, aspects, and advantages of the present invention will become
better understood when the following detailed description is read with reference to
the accompanying drawing in which like characters represent like parts, wherein:
[0008] The Figure is a schematic cross-section of an article treated in accordance with
the description herein.
DETAILED DESCRIPTION
[0009] Approximating language, as used herein throughout the specification and claims, may
be applied to modify any quantitative representation that could permissibly vary without
resulting in a change in the basic function to which it is related. Accordingly, a
value modified by a term or terms, such as "about", and "substantially" is not to
be limited to the precise value specified. In some instances, the approximating language
may correspond to the precision of an instrument for measuring the value. Here and
throughout the specification and claims, range limitations may be combined and/or
interchanged; such ranges are identified and include all the sub-ranges contained
therein unless context or language indicates otherwise.
[0010] In the following specification and the claims, the singular forms "a", "an" and "the"
include plural referents unless the context clearly dictates otherwise. As used herein,
the term "or" is not meant to be exclusive and refers to at least one of the referenced
components being present and includes instances in which a combination of the referenced
components may be present, unless the context clearly dictates otherwise.
[0011] As used herein, the terms "may" and "may be" indicate a possibility of an occurrence
within a set of circumstances; a possession of a specified property, characteristic
or function; and/or qualify another verb by expressing one or more of an ability,
capability, or possibility associated with the qualified verb. Accordingly, usage
of "may" and "may be" indicates that a modified term is apparently appropriate, capable,
or suitable for an indicated capacity, function, or usage, while taking into account
that in some circumstances, the modified term may sometimes not be appropriate, capable,
or suitable.
[0012] The techniques described herein may facilitate the partial or complete removal of
coatings, such as EBC, or other overlying material, from silicon-bearing substrates,
with low mechanical force relative to conventional coating removal techniques. The
term "coating" as used herein simply refers to a quantity of material disposed over
another material; this term does not imply anything about the nature of the material,
in particular as to whether the overlying material forms a continuous layer on the
underlying material. Thus a "coating" as that term is used herein may be continuously
or discretely disposed on the underlying material ("substrate"). The term "silicon-bearing"
is used herein to mean any material that includes, but is not limited to, silicon.
Examples of such materials include without limitation elemental silicon, alloys and
solid solutions that include silicon as a component, and compounds that include silicon.
[0013] Referring to the Figure, an article 100 in accordance with the techniques disclosed
herein generally includes a first material 102 in contact with a second material 104.
In certain embodiments, article 100 is a component of a gas turbine assembly, such
as, for example, a combustion liner, transition piece, shroud, vane, or blade. First
material 102 comprises silicon. In some embodiments, first material 102 includes elemental
silicon; an alloy that includes elemental silicon; a silicide; silicon carbide; silicon
nitride, or a combination that includes one or more of these example materials. In
a particular embodiment, article 100 includes a substrate, with first material 102
disposed over substrate 106, either directly in contact or with one or more interposed
coatings (not shown). For illustrative purposes, in one embodiment, substrate 106
includes silicon carbide, silicon nitride, or combinations of one or more of these;
one example is wherein substrate 106 includes a ceramic matrix composite (CMC). The
CMC includes a ceramic material such as silicon carbide, silicon nitride, or combinations
of one or more of these. A particular example of such CMC is a material that includes
a matrix and a reinforcement phase, where the matrix includes silicon carbide and
the reinforcement phase includes silicon carbide fibers.
[0014] Second material 104 includes an oxide including silicon. This oxide may include,
for example, silica, a silicate, or a combination including one or both of these.
In one example, the oxide of second material 104 is the product of oxidation of one
or more components of first material 102, such as the so-called "thermally grown oxide"
(often abbreviated "TGO") that forms on silicon-bearing coatings and/or substrates
during high temperature service in oxidizing environments. For instance, where substrate
106 includes a CMC, first material 102 may be disposed over substrate 106 as a bond
coat comprising silicon, often elemental silicon; the silicon in the bond coat oxidizes
during service to form a silicon-bearing TGO, which in the parlance of this disclosure
corresponds to second material 104.
[0015] In some embodiments, article 100 further includes a third material 108 disposed over
second material 104, either directly in contact with second material 104 or with one
or more interposed layers (not shown). Third material 108 includes an oxide, such
as one or more of the oxide materials commonly used in the art for thermal barrier
coating (TBC) and/or environmental barrier coating (EBC). Examples include silicates,
such as silicates including one or more rare earth elements; aluminosilicates, such
as aluminosilicate compounds including one or more alkaline-earth elements; and zirconia,
such as yttria-stabilized zirconia. In one illustrative example, substrate 106 includes
a CMC such as a CMC including silicon carbide; first material 102 includes a silicon-bearing
bond coat; and third material 108 includes an oxide top coat commonly used in EBC.
The second material 104, in this example, is a silicon-bearing oxide, such as a TGO,
disposed between the bond coat and the top coat.
[0016] A method for removing a coating, such as the oxide layers of an EBC, or other overlying
material from a silicon-bearing substrate, such as a CMC, includes promoting a reaction
between the silicon of first material 102 with the silicon-bearing oxide of second
material 104 at elevated temperature. This reaction between first material 102 and
second material 104 produces silicon monoxide vapor, which has a very high equilibrium
pressure compared to other relevant reactions, such as thermal decomposition of silica.
The reaction involves equilibrium among 4 phases, including the oxide, the silicon,
the silicon monoxide, and oxygen. Where silicon monoxide vapor product is removed
from contact with the first material 102 and second material 104 rapidly enough to
avoid buildup to equilibrium vapor pressure, and where the partial pressure of oxygen
remains at levels sufficiently low to promote the reaction, the reaction will continue
to run for as long as these temperature and pressure conditions are maintained, until
second material 104 is spent. This reaction essentially vaporizes the connection between
substrate 106 and any material disposed over substrate 106, such as third material
108, allowing this material to become detached from the substrate 106 with little
or no mechanical force. Free edges of article 100, along with surface-connected cracks,
pores, and any other openings in third material 108, allow the SiO reaction product
to escape, preventing buildup of reaction product and accelerating the removal process.
[0017] Based on the above mechanism, one embodiment of a method in accordance with the present
disclosure includes heating article 100 to a processing temperature in an environment
having a partial pressure of oxygen that is less than an equilibrium partial pressure
of oxygen for chemical equilibrium between first material 102 and second material
104 at the processing temperature. In some embodiments, this heating is performed
in a vacuum environment, that is, in an environment having a total pressure that is
less than atmospheric pressure. In some embodiments, the total pressure of the vacuum
environment is less than about 10
-2 torr (1.3 Pa), and in certain embodiments the total pressure is less than about 10
-5 torr (10
-3 Pa). A lower total pressure helps to drive faster reaction rates. Similarly, the
rate of the reaction is also dependent on temperature, but to a stronger degree. A
higher temperature results in faster reaction kinetics. For example, to completely
remove a 20 micrometer thick layer of silica-bearing TGO to a distance of about 1.3
cm (0.5 inch) from a free edge of a coated part, heating in vacuum to a temperature
of 1200 degrees Celsius requires about 32 hours, a temperature of 1300 degrees Celsius
requires about 6 hours, and a temperature of 1400 degrees Celsius requires about 1.2
hours. In some embodiments, the processing temperature is at least about 1200 degrees
Celsius, and in particular embodiments, the processing temperature is at least about
1300 degrees Celsius. Of course, a practical upper limit for temperature may be determined
by the particular circumstances; for instance, if the substrate 106 includes temperature-sensitive
material, such as elemental silicon, it may be desirable to remain below the melting
point of this material to avoid damaging the substrate 106.
[0018] Heating of article 100 to maintain a temperature as described above may be continued
for a time until a desired amount of material removal has taken place. The selected
time depends on several factors, such as the temperature and pressure of the heating
environment, the size of the article to be treated, and the availability of escape
paths for the reaction product vaporizing away from the site of the reaction between
first material 102 and second material 104. If essentially all of at least one of
the reaction products is consumed, then any overlying materials, such as third material
108, may be readily removed from substrate 106 by simply sliding it away from substrate
106 if the geometry allows; in some cases, such as where the overlying material completely
encases substrate 106, or where geometry is complex, the overlying material may have
to be fractured before it can be removed in one or more sections. In some embodiments,
it may not be necessary to completely vaporize first and/or second materials; the
connection they provide between substrate 106 and overlying materials may be degraded
to a point where only a small mechanical force is needed to remove the overlying material,
significantly reducing the risk of damage to the CMC substrate 106. Any convenient
method for removing the overlying material may be applied, such as grit blasting,
water impingement, air impingement, or other appropriately selected method that will
not unduly damage the substrate 106.
EXAMPLES
[0019] The following examples are presented to further illustrate non-limiting embodiments
of the present invention.
[0020] To further illustrate the features described above, a particular embodiment of the
invention is a method for removing a coating from an article. The method includes
heating the article to a processing temperature at least about 1200 degrees Celsius
in a vacuum having a total pressure less than about 10
-2 torr (1.3 Pa). The article 100 includes a substrate 106 comprising a ceramic matrix
composite, the composite comprising silicon carbide, silicon nitride, or a combination
comprising one or both of the aforementioned; a first material 102 disposed over the
substrate 106 and including elemental silicon, an alloy comprising elemental silicon,
a silicide, or a combination comprising one or more of the aforementioned; a second
material 104 in contact with the first material 102, the second material comprising
silica, a silicate, or a combination comprising one or both of the aforementioned,
and a third material 108 disposed over the second material 104, the third material
108 comprising a rare earth silicate, an aluminosilicate, zirconia, or a combination
comprising one or more of the aforementioned. The article 100 is heated at the processing
temperature in the described environment until a desired degree of reaction between
first material 102 and second material 104 has occurred, and then the third material
108 is removed from substrate 106.
[0021] A CMC substrate of silicon-carbide matrix with silicon carbide fiber reinforcement
was coated with a bondcoat of elemental silicon and an oxide topcoat, and the coated
CMC was subjected to about 2000 total hours of exposure to steam at about 1315 degrees
Celsius. The exposed article was then placed in a vacuum furnace and heated to a processing
temperature of about 1300 degrees Celsius for 75 hours. Upon cooling, complete separation
of the topcoat from the substrate was observed. The topcoat was easily slid off of
the surface of the substrate.
[0022] While only certain features of the invention have been illustrated and described
herein, many modifications and changes will occur to those skilled in the art. It
is, therefore, to be understood that the appended claims are intended to cover all
such modifications and changes as fall within the true spirit of the invention.
1. A method for removing a coating from an article (100), the method comprising:
heating an article (100) to a processing temperature, the article (100) comprising
a first material (102) in contact with a second material (104), the first material
(102) comprising silicon, and the second material (104) comprising an oxide comprising
silicon;
wherein the heating is performed in an environment having a partial pressure of oxygen
that is less than an equilibrium partial pressure of oxygen for chemical equilibrium
between the first material (102) and the second material (104) at the processing temperature.
2. The method of claim 1, wherein the environment is a vacuum.
3. The method of claim 1, wherein the environment is a vacuum having a total pressure
less than about 10-2 torr (1.3 Pa).
4. The method of claim 3, wherein the total pressure is less than about 10-5 torr (10-3 Pa).
5. The method of claim 1, wherein the processing temperature is at least about 1000 degrees
Celsius.
6. The method of claim 1, wherein the processing temperature is at least about 1200 degrees
Celsius.
7. The method of claim 1, wherein the processing temperature is at least about 1300 degrees
Celsius.
8. The method of claim 1, wherein the first material (102) comprises elemental silicon,
an alloy comprising elemental silicon, a silicide, silicon carbide, silicon nitride,
or a combination comprising one or more of the aforementioned.
9. The method of claim 1, wherein the oxide comprises silica, a silicate, or a combination
comprising one or more of the aforementioned.
10. The method of claim 1, wherein the article (100) comprises a substrate (106), and
the first material (102) is disposed over the substrate (106).
11. The method of claim 10, wherein the substrate (106) comprises silicon carbide, silicon
nitride, or a combination comprising one or both of the aforementioned.
12. The method of claim 10, wherein the substrate (106) comprises a ceramic matrix composite,
the composite comprising silicon carbide, silicon nitride, or a combination comprising
one or both of the aforementioned.
13. The method of claim 1, wherein the article (100) further comprises a third material
(108) comprising an oxide disposed over the second material (104).
14. The method of claim 13, wherein the oxide of the third material (108) comprises a
rare earth silicate, an aluminosilicate, zirconia, or a combination comprising one
or more of the aforementioned.
15. A method for removing material from an article (100), the method comprising:
heating an article (100) to a processing temperature at least about 1200 degrees Celsius
in a vacuum having a total pressure less than about 10-2 torr (1.3 Pa),
wherein the article (100) comprises
a substrate (106) comprising a ceramic matrix composite, the composite comprising
silicon carbide, silicon nitride, or a combination comprising one or both of the aforementioned,
a first material (102) disposed over the substrate (106), the first material (102)
comprising elemental silicon, an alloy comprising elemental silicon, a silicide, or
a combination comprising one or more of the aforementioned,
a second material (104) in contact with the first material (102), the second material
(104) comprising silica, a silicate, or a combination comprising one or both of the
aforementioned, and
a third material (108) disposed over the second material (104), the third material
(108) comprising a rare earth silicate, an aluminosilicate, zirconia, or a combination
comprising one or more of the aforementioned; and removing the third material (108)
from the substrate (106).